Add ability to pass Serial Number and Common Name

[ellgreen]

Thanks for this library, we're finding it useful! 😄

Just wanted to add the ability to set a serial number and common name when creating new key pairs.

Currently we have some tests that need distinct values for these which is making it a little bit harder to test what we need.

Summary by CodeRabbit

• New Features
  • Added support for SerialNumber and CommonName in certificate generation, enhancing configurability.
• Bug Fixes
  • Improved handling of serial numbers and common names in the certificate generation process.
• Tests
  • Expanded test coverage for KeyPairConfig to validate new features and ensure correct functionality, including new test scenarios for serial numbers and common names.